Thermally insulating jacket and related process

ABSTRACT

Improved process for evacuating the thermally insulating jacket of a dewar having an inner wall and an outer wall, with the inner space between said walls completely or partially filled with an insulating material, containing also a moisture sorbing material and a getter material, in which said moisture sorbing material is a chemical drying agent.

This application is a continuation-in-part of U.S. Ser. No. 08/038,643filed Mar. 29, 1993, now abandoned which in turn is a Continuation inthat it discloses and claims subject matter in addition to thatdisclosed in prior, U.S. Ser. No. 07/979,326 entitled “IMPROVED PROCESSFOR EVACUATING A THERMALLY INSULATING JACKET AND IN PARTICULAR THEJACKET OF A DEWAR OR OF ANOTHER CRYOGENIC DEVICE” filed Nov. 20, 1992,abandoned.

BACKGROUND OF THE INVENTION

1) Field of the Invention

The present invention relates to an improved process for evacuating athermally insulating jacket and in particular the jacket of a dewar orof another cryogenic device, like for instance dedicated pipings for thestorage and/or the transport of cryogenic gases such as nitrogen,oxygen, hydrogen, helium, argon etc. or of other substances requiringthe maintenance of a temperature different from room temperature,usually lower, the insulating features of said jackets being notoriouslyreached by means of vacuum and of an insulating material.

The new process is particularly advantageous, as it allows said jacketto be put in condition to work in a very short time.

2) The Prior Art

It is known, according to the common practice, to use for the purposesabove dewars or pipings having a vacuum jacket in order to reach anadequate thermal insulation. As a further thermoinsulating measure,there is usually inserted in the jacket an insulating material likeglass wool, expanded organic polymers (e.g. polyurethane and resinshaving various compositions) and, very frequently, the so called“multilayers”. These latter are consisting of alternated sheets madefrom organic polymers (such as polyolefines), preferably showing across-linked structure, and of aluminized plastics, obtained forinstance, as it is known, by coating a plastic film with aluminium bymeans of evaporation under vacuum.

It is also known that the vacuum, in said jacket, has the tendency todegrade along the time, because of the outgassing of the innercomponents and even of the walls (namely by emission of gaseous specieslike CO, N₂, H₂, H₂O, O₂ etc.) and owing to possible “leaks” (namelypenetration of an atmospheric gas). In order to maintain the vacuum,there is normally utilized a gas sorbing material, placed in the jacketand generally consisting of zeolites, molecular sieves, silica gel,activated carbon (charcoal) and other sorbing material having physicalaction. These substances have to be kept at a very low temperature, e.g.the temperature of liquid nitrogen or a lower one, in order to allow thesorption of the most part of such gaseous species. Moreover, thephysical sorbing materials hereinabove do not effectively sorb hydrogen.Therefore it was required in the past for instance, to insert in thejacket palladium oxide, as disclosed by GB-A-921273. Pd oxide in factconverts hydrogen into water, according to the reaction:PdO+H₂→Pd+H₂O (∝)

Water is then physically sorbed by the zeolites or by the other physicaladsorbents present in the jacket.

A first drawback, coming from the use of these techniques, is due to thefact that the preparation of the insulating jacket of the dewars or ofpipings above requires a very long time, because the activation of thephysical adsorbents (zeolites and the like) requires an extended thermaltreatment under pumping, which can even last several days.

The process length is mainly determined by the necessity of reducing asfar as possible the residual amount of water contained in the physicaladsorbents and in the insulating materials. A shorter time could bereached by drastically increasing the temperature of the thermaltreatment of the system (e.g. beyond 150°-200° C.), but frequently thistreatment can not be carried out because the involved materials are notconsistent with such temperatures or because of practicality grounds orof grounds bound to the process economics.

A second drawback comes from the physical nature of the sorption, whichmakes the reaction reversible. Therefore, for instance, as a dewar isemptied and its inside is at room temperature, the sorbed gases arere-emitted thus giving rise to a certain pressure in the jacket.Consequently, during the filling of the inner vessel (of the dewar or ofthe piping), already returned to room temperature, there is observed avigorous boiling, with a gas loss, because of the worsened insulationconditions. These are re-established after a certain time, once thezeolites have been cooled down to cryogenic temperature and have sorbedagain the gases. Such a problem, known as “boil-off” or reboiling, is aserious one, especially in the case of liquefied gases like H₂ and He,which are relatively expensive and endowed with a low heat ofevaporation. It should also be underlined the fact that a few of theknown materials used so far do react with the liquefied gas, with whichthey can occasionally get into contact, as soon as there is a leakagefrom the inside of the vessel, e.g. because of the formation of cracks.PdO in particular cannot be utilized with liquefied H₂, because ofpossible explosions, in the case of breaking of the inner wall. The sameoccurs in the case of liquid O₂, when activated carbon (charcoal) isused.

It was suggested by C. Boffito et al., in J. Vac. Sci. Technol. A5 (6),3442 (1987), to use a gettering material, based on a Zr-V-Fe alloydisclosed in GB-A-2043691. This material is actually solving a few ofthe problems hereinabove, as it can chemically sorb the different gases,responsible for the degrading of the vacuum, and especially hydrogen.

The insulating material however gives rise, during the manufacturingprocess, to a heavy release of water, which can considerably jeopardizethe effectiveness of the sorption, with respect to the other gaseousspecies, all along the life of the device.

It is therefore a first object of the present invention to provide aprocess allowing to shorten the time required for evacuating and makingready the insulating jacket of a dewar or of an other cryogenic device,such as for containing and/or transporting liquefied gases.

Another object of the present invention is to provide a process of thekind hereinabove, free from said “boil-off” problems, during the fillingof the vessel with liquefied gases, onche the vessel has got empty andafter it has come back to room temperature.

A further object of the present invention is to provide a processallowing the elimination of the hazard bound to some of the materialsused for the maintenance of vacuum, in particular palladium oxide andactivated carbon (charcoal). These materials can react in fact in anexplosive way with hydrogen and oxygen, respectively, if either of thetwo gases are present in a liquefied form, inside the vessel to beinsulated and if such a vessel is undergoing a breakage.

Still another object of the present invention is to provide a processfor obtaining an insulated device (dewar or piping) granting aneffective chemical pumping, with respect to the undesired gases, allalong the life of the same device.

DISCLOSURE

These objects can be accomplished by an improved process for evacuatingand making ready a thermally insulating jacket and in particular thejacket of a dewar or of another cryogenic device, having an inner walland an outer wall and having the inner space between said wallscompletely or partially filled with an insulating material, wherein saidinner space also contains a moisture sorbing material and a gettermaterial, characterized in that said moisture sorbing material is achemical drying agent, preferably selected from the sorbing materialsshowing an H₂O vapor pressure lower than 1 Pa and in particular frombarium oxide, strontium oxide, phosphorus pentoxide and mixturesthereof.

When no liquid hydrogen is present inside or outside the jacket to beevacuated, said inner space may also advantageously contain a hydrogenconverter like for instance the oxides of a few noble metals (palladium,iridium, osmium, rhodium, ruthenium); most preferred hydrogen converteris palladium oxide (PdO) optionally in admixture with said Ba oxide.

According to a preferred very effective embodiment, the new processcomprises the following subsequent steps:

-   -   A. the inner space of the jacket is first evacuated down to a        pressure lower than 100 Pa by means of a vacuum pump;    -   B. said inner space is contemporaneously exposed to said        chemical drying agent, and optionally to said hydrogen converter        (provided no liquid H₂ is present inside or outside the jacket)        while keeping the getter in an inactivated form;    -   C. said inner space is further evacuated down to a pressure        lower than 5 Pa, by means of a vacuum pump;    -   D. said getter is activated; and    -   E. the jacket is isolated from the pump, by sealing the        connection between said pump and the inner space of the jacket.

Optionally the pumping under item A hereinabove can be discontinuedduring the operation under item B, and during the operation under item Aand B the inner wall of said dewar, or of another cryogenic device, iskept hot, at a temperature not higher than 150° C. and preferably 120°C., thus promoting the release of water from the insulating material.

The operation under item B does normally last no more than 48 h andpreferably from 2 to 48 h instead of the several days required in thepast.

Moreover said chemical drying agent and said getter are preferably lyingin separate zones, contrary to the previous teachings of the prior art,against the outer wall of said dewar or of other cryogenic device.

As to said getter, it may consist of the alloys which can be activatedat a relatively low temperature, like those based on barium and/orzirconium and preferably the alloys Ba-Li as disclosed in the EuropeanPatent Application No. 92830186 in the name of the same applicant, inparticular the alloy having raw formula Ba Li₄.

In other more detailed words the new improved process allows to evacuateand to make ready a thermal insulating jacket of a cryogenic device(dewar, transfer or storage pipings and so on) for the storage and/orthe transport of a stuff which has to be kept at a temperature differentfrom room temperature, in particular lower. Such a process allows tosorb the gases originated during the same process and during thesubsequent life of the cryogenic device, by combining the action of aparticular (chemical) water adsorbing agent, prevailingly acting in thefirst stages of the process (by providing a water pumping “in situ”,which accelerates the same process) along with the action of a getter,activated only in a subsequent step, which is prevailingly acting as achemical sorbing material with respect to the other gases different fromwater, like for instance O₂, N₂, CO, H₂, etc.

The invention is hereinafter described more clearly with reference tothe following drawings, which are supplied for merely illustratingpurposes, without limiting in any way the scope of the same invention,in which:

FIG. 1 shows a schematic sectional view of a commonly used metal dewarfor the storage of liquefied gases, having an insulating jacketaccording to the invention;

FIG. 2 shows a graph reporting the results of the tests carried out bymeans of said dewar of FIG. 1;

FIG. 3 shows a preferred arrangement of the drying agent and of thegetter inside the vacuum jacket;

FIG. 4 shows a schematic sectional view of a commonly used metal pipe,for the transportation of liquified gases, having an insulation jacketevacuated according to the invention; and

FIG. 5 shows a graph reporting the results of the tests carried out bymeans of the metal pipe of FIG. 4.

The series of operations hereinbelow, embodying the process according tothe invention, is described referring to a vessel like the dewar of FIG.1.

As it is known, the dewar 1 consists of an inner container or vessel 2,preferably made from metal, e.g. steel, defining an inner volume oruseful space 3, suitable to contain a liquefied gas, which cancommunicate with the surroundings by means of a “neck” 6, normallyclosed but not sealed. An outer wall or mantel 4 defines, along with theinner wall 2, a jacket 5, partially filled, at least in the portionsurrounding the inner wall 2, with an insulating material 9, preferablythe “multilayer” type herein above. The jacket 5 can communicate with anouter pumping system (not shown in the drawings) by means of aconnecting fitting 8 and a valve 7 for switching off or disabling thepump.

According to the present invention, there are inserted in the jacket 5 achemical moisture adsorber 10 and a gettering material (11) (andoptionally, in certain cases pointed out hereinabove, a hydrogenconverter) lying in separate zones, against the outer wall 4, contraryto the known prior art, describing the getter material to be positionedagainst the inner wall, at a lower temperature. Then a first pumpingstep of the jacket 5 starts, through the tubular fitting 8, untilreaching a pressure of 100 Pa or lower, which is anyhow requiring only afew minutes. The valve 7 is subsequently closed, thus isolating thevessel wall from the pumping apparatus, and the water sorbing material10 exerts, for a time between 2 and 48 h, a selective pumping actionwith respect to the water vapor outgassed from the insulating material9.

During this step, the valve 7 can be also kept open all along saidinduction time. In such a case, however, the drying material 10 isalways sorbing the most part of the water vapor, because the action ofthe pump is limited by the flow conductance allowed by connection pipefitting 8. Again in this phase of water vapor sorption (with or withoutany pumping from outside) the inner wall 2 can undergo a smooth heating,not above 150° C., for istance by penetration into the space 3 of hotair or of hot water, in order to accelerate the removal of the watervapor from insulating material 9, in particular from the innermostlayers, lying near the wall 2, more than the other layers.

After the induction time, pumping is started again, should it have beendisabled, by re-opening the valve 7, down to a pressure of 5 Pa or less.At this point the getter 11 is activated, e.g. by means of a heatgenerating device arranged at the outside, at a location correspondingto the inside positioning of the same getter. The heating can simplyoccur by using a flame, a hot air gun, an electric resistance or othersimilar means. The temperature to be reached depends on the kind ofgetter selected for the use. Getters which can be activated at a low orvery low temperature are preferred, like the Ba-Li alloys disclosed inthe European Patent Application No. 92830186, inserted in a blistersupplied with a thermoretractable cover, according to the EuropeanPatent Application No. 92830185 also in the name of the same Applicant.Getter materials of this kind do not require a heating at a temperaturehigher than 120° C. in order to be activated.

Eventually, during the last step of the process, the jacket 5 isdefinitely isolated from the outside by closure of the valve 7,isolation of the pump and sealing (e.g. by means of a “pinch-off”) incorrespondence to the tubular fitting 8.

The same getter is working in a much better way if the sorbed gas doesnot contain water. This is the reason why it is suggested to let thegases get into contact first with the powerful drying agent, such as BaO(and optionally with a hydrogen converter), and then, in a separatezone, with the getter, e.g. Ba Li₄.

More in detail, and according to the embodiment of FIG. 3, said chemicaldrying agent 10 and said getter 11 can be arranged in a container 12subdivided into an inner and an outer zone by a porous septum 13,wherein the inner zone 14 contains said getter; the outer zone 15 iscommunicating with the space containing said insulating material 9 andcontains said chemical drying agent 10, which prevents the passage ofthe water vapour through said septum and towards said getter 11.

Container 12 may be vertical box having an opening at its top and aplanar (e.g. horizontal) septum or a toroidal box, wrapping for instancethe inner vessel of a dewar or of a cryogenic piping, having a radial ora planar (e.g. horizontal) septum.

The same container can be also a rigid, semirigid or flexible box, madefrom a substantially water-free material, preferably from metal, glass,ceramics or combinations thereof. It may have a toroidal shape, but alsodifferent shapes in cross section could be adopted, e.g. circular,square, rectangular, triangular, elliptical, oval, lobe-shaped and ofsimilar configuration.

The following examples are supplied for illustrating purposes and do notlimit in any way the scope of the invention.

EXAMPLE 1 (COMPARATIVE)

This example has the purpose to show the behaviour of a dewar like theone illustrated in FIG. 1, having a vacuum jacket prepared withoututilizing the technique according to the present invention.

Said vacuum jacket, having a volume of 36 L and containing 500 g of amultilayer insulating material, consisting of polyolefinic tapesalternated with aluminized polyester tapes (traded as MYLAR tapes), wasconnected to an outer vacuum pump, of the rotary and turbomoleculartype, while maintaining the pumping for 5 h. Subsequently the dewar wasisolated and the pressure increase was started to be recorded along thetime.

The results of these tests are represented by the line 1 of FIG. 2. Line2, on the contrary, is showing the pressure increase merely due to thepenetration of air into the jacket through the leaks present in thedewar, of the order of 6×10⁻⁷ Pa m³/s, on the basis of a measurementcarried out by means of a mass spectrometer. The tested device was quitewell representing the commonly used actual devices; however, in order toperform an accelerated test, there was produced the air leakage asindicated hereinabove, at least 5000 times greater than the valueconsidered as allowable in the case of such an exploitment (normally ofthe order of 10⁻¹⁰ Pa m³/s). It was thus possible to reproduce in ashort time the effects of a long lasting leakage. The difference betweenline 1 and line 2 is due to the gases, prevailingly water, released bythe outgassing of the inner insulating material.

EXAMPLE 2

The test of example 1 was repeated, while inserting however in thevacuum jacket, in the zones corresponding to the drying agent 10 and tothe getter 11, respectively 10 g of BaO and 10 g of an alloy Ba Li₄,according to the European Patent Application 92830186, in the form ofgranules (small pellets) arranged in the thermoretractable containers(vessels) as disclosed in the European Patent Appliction 92830185.

The vacuum jacket was connected to the outer pump and submitted to ashort pumping for 10 minutes. Then the system was isolated from the pumpby closing the valve 7. Under such conditions, there was an inductiontime of 24 h, followed by a second stage (re-pumping of the system), bypreviously opening the valve 7, while contemporaneously activating thegetter up to 120° C. by means of a heating device, arranged in anoutside location, corresponding to the inside positioning of the samegetter, while utilizing to this purpose a hot air gun. The activationtime resulted to be 25 minutes; thereafter the system was isolated andrecording of the pressure versus time was caused to start. The resultsof these tests are plotted as line 3 on FIG. 2.

EXAMPLE 3

This example has the purpose to show the behaviour of the stainlesssteel pipe, to be used for transferring liquid nitrogen, illustrated inFIG. 4, insulated through a vacuum jacket, evacuated according to thetechnique of the present invention.

Said vacuum jacket had a volume of 20 liter and contained approximately500 g of a multilayer insulation material, consisting of polyolefinictapes alternated with aluminized polyester tapes (traded as MILAR),wrapped around the inner pipe.

The following materials were also contained in the jacket:

-   -   10 g of a BaLi₄ alloy according to the European Patent        Application 92830186, in the form of granules (small pellets)        arranged in a thermo-retractable container 17 as disclosed in        the European Patent Application 92830185, placed in the getter        compartment 16;    -   20 g of BaO (18), freely dispersed in the jacket; and    -   0,75 g of PdO granules, wrapped in a porous metallic packet 19,        fixed onto the wall of the getter compartment 16.

The vacuum jacket was connected, by means of valve 20, to an outerpumping system, consisting of a diffusion and rotary pump, and checkedfor tightness, by means of a leak detector having a sensitivity of5×10⁻¹² Pa m³/s.

Pumping was then maintained for 15 h, blowing the inner pipe with hotair at 100°-120° C., to promote the degassing of the multilayerinsulating material and, finally, the getter compartment 16 was heatedat 120° C. by means of an external tape heater, thus causing the getteractivation.

After 30 min. of activation, the jacket was isolated from the pumpingsystem, by closing valve 19, and the pressure was recorded versus thetime.

The results of this test are plotted in FIG. 5.

DISCUSSION

By comparing lines 1 and 3 of FIG. 2, it can be observed that applyingthe technique according to the invention allows for a substantialreduction of the pressure increase accurring in the cryogenic deviceafter its sealing. A few analytical tests, carried out by means of amass spectrometer, showed that the combined use of the two materials,drying agent and getter, does not only allow to quantitatively sorb thewater outgassed by the insulating material, but also to face the leaksin the device. Further it should be appreciated that the tests wereperformed under accelerated conditions, namely under a more drasticcondition, as to the load of atmosheric gases, with respect to thoseoccurring in the practice. Therefore an accelerated test lasting 360 h(515 days) is equivalent to at least 20 years of actual working.

In the case of a cryogenic device, in which the leak rate issubstantially lower and normally less than 10⁻¹⁰ Pa×m³/s, the pumpingaction exerted by the getter materials leads to an appreciable pressuredrop after the sealing of the jacket, as shown by the line in FIG. 5.

It is understood that optional additions and/or changes can be carriedout by those skilled in the art, with respect to the operativeconditions hereinabove, without departing from the scope and spirit ofthe claimed invention.

1. A process for producing an improved thermally insulating jacket,having an inner wall and an outer wall, and having an inner spacebetween said walls containing an insulating material, wherein said innerspace also contains a moisture sorbing material and a getter material,wherein said moisture sorbing material is a moisture sorbing material,having a H₂O vapor pressure lower than 1 Pa at room temperature,characterized by the following steps: A. evacuating the inner space ofthe jacket down to a pressure lower than 100 Pa by means of a vacuumpump having a connection between the pump and the inner space of thejacket; B. exposing said inner space contemporaneously to said moisturesorbing material while keeping the getter in an inactivated form; C.evacuating said inner space farther, down to a pressure lower than 5 Pa,by means of the vacuum pump; D. activating said getter; and E. isolatingthe jacket from the vacuum pump, by sealing the connection between saidvacuum pump and the inner space of the jacket.
 2. A process according toclaim 1 characterized in that during the exposing of step B, theevacuating according to Step A is discontinued.
 3. A process accordingto claim 2 wherein step B lasts from about 2 to about 48 hours.
 4. Aprocess according to claim 1, characterized in that during the Steps Aand B, the inner wall is kept hot at a temperature of not higher than150° C. thus promoting the release of water from the insulationmaterial.
 5. A process according to claim 4 wherein step B lasts fromabout 2 to about 48 hours.
 6. A process according to claim 1,characterized in that the step B lasts for up to 48 hours.
 7. A processaccording to claim 1 characterized in that said moisture absorbingmaterial and said getter are lying, in separate locations, against theouter wall of said jacket.
 8. A process according to claim 7characterized in that said moisture sorbing material and said getter arearranged in a container subdivided into an inner zone and an outer zoneby a porous septum, wherein: the inner zone contains said getter; theouter zone is communicating with the inner zone containing saidinsulating material and contains said moisture sorbing material whichprevents the passage of water vapour through said septum and towardssaid getter.
 9. A process according to claim 8 characterized in thatsaid container is a vertical box having an opening at its uppermostportion and a planar septum.
 10. A process according to claim 8characterized in that said container is a toroidal box having a radialor planar septum.
 11. A process according to claim 9, characterized inthat said septum is horizontal.
 12. A process according to claim 8,characterized in that said container is a rigid, semirigid or flexiblebox.
 13. A process according to claim 8, characterized in that saidcontainer is made from a substantially water-free material, selectedfrom the group consisting of metal, glass, ceramics and combinationsthereof.
 14. A process according to claim 1, characterized in that saidgetter material is an alloy having the formula BaLi₄.
 15. A processaccording to claim 1 wherein step B lasts from about 2 to about 48hours.
 16. A process according to claim 1, characterized in that saidseptum is horizontal.
 17. A jacket of claim 1 wherein said moisturesorbing material is selected from the group consisting of barium oxide,strontium oxide, phosphorous oxide, and mixtures thereof.
 18. Animproved thermally insulating jacket, having an inner wall and an outerwall, and having an inner space between said walls completely orpartially filled with an insulating material, wherein said inner spacealso contains: A. a moisture sorbing material selected from the groupconsisting of barium oxide, strontium oxide, phosphorous oxide, andmixtures thereof; and B. a getter material which is an alloy of theformula BaLi₄.
 19. An improved thermally insulating jacket, having aninner wall and an outer wall, and having an inner space between saidwalls completely or partially filled with an insulating material,wherein said inner space also contains: A. a moisture sorbing materialselected from the group consisting of barium oxide, strontium oxide,phosphorous oxide, and mixtures thereof; and B. a getter material whichis an alloy of the formula BaLi₄; and C. a hydrogen converter selectedfrom the group consisting of osmium oxide, iridium oxide, rutheniumoxide, rhodium oxide and palladium oxide.
 20. An improved thermallyinsulating jacket, having an inner wall and an outer wall, and having aninner space between said walls completely or partially filled with aninsulating material, wherein said inner space also contains: A. amoisture sorbing material which is barium oxide; and B. a gettermaterial which is an alloy of the formula BaLi₄; and C. a hydrogenconverter which is palladium oxide.
 21. A method for producing athermally insulating jacket, comprising: evacuating an inner spacedefined by inner and outer walls to form a vacuum; filling an innerspace of the jacket with insulating material, the inner space beingdefined by an inner wall and an outer wall; providing for absorbing bothwater vapor and at least a second type of gas or vapor from the innerspace with a getter; providing for sorbing water vapor with a watersorbing material; positioning the getter and water sorbing material inthe inner space in a container that is impervious to water vapor;subdividing the container into an inner zone and an outer zone, thegetter being positioned in the inner zone of the container and the waterabsorber filling the outer zone of the container: placing the outer zoneof the container in communication with both the inner space and theinner zone of the container, and placing the inner zone of the containerin communication with only the outer zone of the container so that thewater absorber in the outer zone prevents the water vapor from reachingthe getter; evacuating the inner space to a predetermined level ofpressure; and sealing the inner space with the container therein so thatthe water sorber continues sorbing the water vapor and the gettercontinues absorbing the second gas or vapor.